So-called radio-frequency identification systems, or “RFID” systems for short, are being used ever more widely. In principle, they may comprise two components, specifically a so-called transponder and a detection appliance, which is normally a combined reader/writer.
The transponder may be linked to an object which is intended for identification, with the detection appliance carrying out this identification by non-contacting means. The detection appliance typically comprises a radio-frequency module, with a transmitter and receiver, a monitoring unit and a coupling element for the transponder. The transponder, which represents the actual data storage medium for an RFID system, normally comprises a coupling element as well as an electronic component, a so-called chip. No specific voltage supply is normally provided in the transponder. Its behavior is passive outside the response area of the detection appliance. The energy which is required for operation of the transponder is transmitted by the coupling unit to the transponder without contact.
The communication between the transponder and the detection appliance normally takes place by means of a modulated data signal.
Load modulation is normally used for data transmission, in which the magnitude of the level of the unmodulated signal is reduced on modulation.
High energy levels can be received by the transponder when power is supplied, particularly in the short-range area, and this can result in heating of the circuit arrangement, or of parts of it.
The power loss resulting from the field is proportional to the antenna voltage. However, load modulation as is normally used for RFID transponders is impossible if the antenna voltages are low. Load-reduction modulation, in which the level of the unmodulated signal is less than the level of the modulated signal, is therefore used, with switching taking place between this level and another level on modulation.